1. Field of the Invention
The present invention relates to a vibration-sensing device, which includes a support member fixed to an outside fixation member, and vibrating tines or a vibrating member of a tuning fork shape vibrating along a predetermined direction in a plane and supported by the support member. The invention also pertains to a method of adjusting such a vibration-sensing device, as well as to an angular velocity sensor for detecting an angular velocity by taking advantage of such a vibration-sensing device.
2. Description of the Related Art
When a rotating force is applied to a vibrating member vibrating along a predetermined direction, for example, along an X axis in the plane which contains rectangular coordinate axes (X-Z plane) and the vibrating member rotates round an Y axis perpendicular to the X-Z plane, the angular velocity of rotation causes the vibrating member to receive a Coriolis force in the direction of Z axis. The Coriolis force depending upon the angular velocity is measured indirectly as a deflection displacement of the vibrating member or as a stress on the vibrating member or directly by means of the piezoelectric effect of piezoelectric elements, and the angular velocity of the vibrating member is then calculated from the Coriolis force. A vibration-sensing device having such a vibrating member vibrating along the X axis is applied to the angular velocity sensor, which is mounted on a vehicle or another object to detect the yaw rate observed by turning the vehicle. The angular velocity sensor is also mounted on the body or arm of an industrial robot to detect the yaw rate observed on the body or arm accompanied by a shift of the robot and control the orientation of the robot. For example, a vibrating angular velocity sensor with a vibration-sensing device having a tuning fork-shaped vibrating member has been proposed in the U.S. Pat. No. 4,538,461.
The vibration-sensing device applied to the vibrating angular velocity sensor disclosed in the U.S. Pat. No. 4,538,461 has a tuning fork-shaped crystal vibrating member, where a pair of vibrating tines are joined with each other on one end to form a tuning fork. The tuning fork-shaped crystal vibrating member (hereinafter referred to as the first tuning fork-shaped vibrating member) is connected with a pivot at the joint of the two vibrating tines. Both ends of the pivot are fixed to a frame by means of support beams arranged perpendicular to the pivot. The pivot is provided with a dummy reaction mass element in response to torsion vibration of the first tuning fork-shaped vibrating member, whereas a vibration adjusting mass element is attached to one end of each vibrating tine of the first tuning fork-shaped vibrating member.
Vibration-driving electrodes for driving vibrations of the vibrating tines along the X axis are mounted on the respective vibrating tines of the first tuning fork-shaped vibrating member, whereas detecting-electrodes for detecting torsion vibrations are mounted on the pivot. The vibrating angular velocity sensor thus constructed detects the angular velocity in the following manner. Alternating voltages are continuously applied to the vibration-driving electrodes to give the vibrating tines with the driving electrodes mounted thereon plane vibrations along the X axis. When an angular velocity acts on the sensor while the vibrating tines are under the condition of plane vibration, the angular velocity causes torsion vibration in the sensor, which is detected as a voltage output from the detecting electrode.
When an angular velocity is constantly applied to the sensor while the vibrating tines of the first tuning fork-shaped vibrating member are under the condition of plane vibration, the mass balance of plane vibration and torsion vibration of the two vibrating tines of the first tuning fork-shaped vibrating member is of great importance for the stable torsion vibration of the sensor, as is known well. In the vibration-sensing device disclosed in the U.S. Pat. No. 4,538,461, the mass balance of vibrations is attained by appropriately designing the shape of the vibration adjusting mass element attached to the end of each vibrating tine or controlling the mass of the vibration adjusting mass element.
Problems as described below, however, arise in the vibrating angular velocity sensor proposed by the U.S. Pat. No. 4,538,461.
In such a vibrating angular velocity sensor, it is essential to improve the detection sensitivity of the angular velocity, as well as to stabilize the torsion vibration driven on the sensor. As is already known, for the improved detection sensitivity of the angular velocity, in addition to the mass balance of vibrations of the vibrating tines, it is required to set a predetermined relationship between the resonance frequency of plane vibration driven in the direction of X axis and the resonance frequency of detected vibration. For example, both the resonance frequencies are made substantially identical with each other. In the sensor disclosed in the U.S. Pat. No. 4,538,461 for detecting the torsion vibration, it is required to adjust the resonance frequency of plane vibration along the X axis and the resonance frequency of torsion vibration along the Z axis driven by the application of angular velocity so that the resonance frequencies might become substantially identical with each other. It is known that the detection sensitivity of the sensor, as well as the temperature characteristics, the S/N ratio, and the stability, depends upon the degree of adjustment. However, the following difficulties arise in the adjustment of frequency.
Mass control on the ends of vibrating tines is known to be effective for the adjustment of frequency of the first tuning fork-shaped vibrating member. Since the end of each vibrating tine is under a large vibration displacement, the mass control on the ends of vibrating tines results in a significant variation in resonance frequency of the first tuning fork-shaped vibrating member. The mass control on the ends of vibrating tines for adjusting the resonance frequency of plane vibration of the vibrating tines of the first tuning fork-shaped vibrating member to a target resonance frequency, that is, the resonance frequency of torsion vibration, leads to an undesirable variation in resonance frequency of torsion vibration. The variation in resonance frequency of torsion vibration is ascribed to the properties on the ends of vibrating tines of the first tuning fork-shaped vibrating member, which have a large vibration displacement in driven plane vibration as well as a large vibration displacement in torsion vibration of the vibrating tines.
As described above, the mass control on the ends of vibrating tines varies both the resonance frequencies of plane vibration and torsion vibration generated by the application of angular velocity. The degree of variation for plane vibration is not identical with that for torsion vibration. It is accordingly difficult to independently adjust the resonance frequency of driven plane vibration and the resonance frequency of torsion vibration by the mass control on the ends of vibrating tines. A complicated adjustment process, for example, monitoring variations in both the resonance frequencies while one of the resonance frequencies is adjusted through the mass control, is thus required to set the difference between both the resonance frequencies within a predetermined range. Such adjustment process including the mass control and the monitor of both the resonance frequencies requires the skill of workers and consumes much labor and time. The process also has difficulties in adjusting the resonance frequencies to desired values with high precision. This prevents sensors of high sensitivity from being manufactured at high yield. These problems are not characteristic of crystal but are observed in other materials including various metals like stainless steel, iron-nickel alloys, and identity elastic alloys, and dielectrics like piezoelectric elements (PZT).
A vibrating angular velocity sensor shown in FIG. 12 of JAPANESE PATENT LAID-OPEN GAZETTE No. H7-113645 has an element of certain form similar to the dummy reaction mass element shown in the U.S. Pat. No. 4,538,461. In the sensor of JAPANESE PATENT LAID-OPEN GAZETTE No. H7-113645, however, the element (represented by the numeral `83` in FIG. 12) having a shape similar to the dummy reaction mass element is provided with wirings to electrodes of vibrating tines and fixed to an outside member. Although having a similar shape, the element 83 does not function as the dummy reaction mass element in the U.S. Pat. No. 4,538,461.
Other problems as described below also arise in the vibrating angular velocity sensor proposed by the U.S. Pat. No. 4,538,461.
The detecting electrodes for detecting torsion vibration generated by the application of angular velocity are formed on the side faces of the pivot used for supporting the first tuning fork-shaped vibrating member and divided vertically along the Z axis. In the sensor having such detecting electrodes, when an angular velocity acts on the sensor to generate Coriolis forces, the pivot with the detecting electrodes receives a torsional force in the direction of X axis. The detecting electrodes detect a stress applied onto the crystal due to the torsional force. The stress detected is a shearing stress .tau.yz of the pivot and has a term depending upon the piezoelectric constant of crystal given by the matrix expressed as: ##STR1##
In the crystal vibrating angular velocity sensor of the U.S. Pat. No. 4,538,461, the piezoelectric constant corresponding to the shearing stress .tau.yz generated by the torsional force on the pivot in the electric field of Y or Z direction is defined by the entry in the second row and the fourth column (e24) or in the third row and the fourth column (e34) in the matrix of Equation 1. Both the entries are, however, equal to zero, and the piezoelectric constant-depending term in the torsional force-based shearing stress detected by the detecting electrodes of the sensor becomes inevitably equal to zero. This angular velocity sensor accordingly can not detect the torsional force-based shearing stress .tau.yz precisely, thereby having difficulty in improving the detection sensitivity.